The Tamil calendar is a derivative of the old Hindu solar calendar and is based on the sidereal year (i.e. the time taken for one revolution of the Earth around the sun, or the mean time taken by the sun to return to the same position relative to the background of the fixed stars in the sky). In contrast, most other languages of India (Telugu, Hindi) use lunar calendars.

The Tamil new year usually falls in mid-April and the calendar consists of twelve months. Unlike the Gregorian calendar, the number of days in a given month can vary between years. Moreover, Tamil months may even have 32 days. For example, the month of Vaikasi had 32 days in 1996 and 31 days in 1998. Similarly, Aani had 31 days in 1996 and 32 days in 1998.

The Tamil calendar also follows a seven-day week. The names of the seven days are analogous to those used in most other calendars i.e. they correspond to the same celestial bodies Sun, Moon, Mars, Mercury, Jupiter, Venus, and Saturn, in that order.